1. Field of the Invention
The present invention relates to a method of fabricating an ion sensitive field effect transitor (ISFET) with a Ta.sub.2 O.sub.5 hydrogen ion sensing membrane, and more particularly to a method of fabricating a pH-ISFET with a Ta.sub.2 O.sub.5 hydrogen ion sensing membrane having high sensitivity and stable operating characteristics.
2. Description of the Prior Art
ISFETs have been developed as a kind of semiconductor ion sensor manufactured by utilizing the manufacturing process of semiconductor integrated circuits and have such characteristics that they are very small in size and light in weight, and thus they can be normally used as ion sensors for in-vivo measurements, being adapted for insertion into the human bloodstream.
Further, the ISFETs have the merits like economic mass production, easy standardization, etc., and thus the ISFET ion sensors can be substituted for existing expensive ion selective electrodes (ISEs). At present, such ISFETs are widely used in the fields of medical treatment, analytical chemistry, environmental management, etc.
The ISFET was firstly reported by P. Bergvelt in 1970, as a result of his preliminary research for an ion sensitive solid-state device for neurophysiological measurements and thereafter, the research was concretely and practically done by Matsuo, Wise, et al.
Table 1 shows sensing characteristics of hydrogen ion sensing membranes of the several pH-ISFETs. According to Table 1, the SiO.sub.2 membrane has low sensitivity as well as severe drift, while the ZrO.sub.2 membrane has a narrow range of pH measurements, and thus they are not generally used.
In comparison with these membranes, the Si.sub.3 N.sub.4 membrane can easily be deposited on the gate of the ISFET as a chemically stable dielectric layer, and thus the current leakage through its gate and the hydration in its surface can be reduced, thereby resulting in a stable sensing operation.
However, deterioration in sensing characteristics may be expected in Si.sub.3 N.sub.4 film depending upon its stoichiometric composition ratio under the specific film depositing condition and storage condition in case that the film is made through low pressure chemical vapor deposition (hereinafter, referred to as LPCVD) process. In other words, under the influence of surface oxidation and non-stoichiometric composition in the film surface, the undesirable hydration may be caused, thereby lowering its sensitivity and greatly increasing the drift.
In order to solve the problem of such disadvantages of Si.sub.3 N.sub.4 membrane, researches of Al.sub.2 O.sub.3 and Ta.sub.2 O.sub.5 thin films which are metal oxides, have been actively carried out.
The Ta.sub.2 O.sub.5 thin film has much better sensing characteristics than those of other ISFET hydrogen ion sensing membranes and further, it has the advantage that the deterioration of its characteristics due to its surface oxidation are not found, differently from those of the Si.sub.3 N.sub.4 membrane even when it is exposed to the open air. However, it has the disadvantage that its sensing characteristics would be affected by the film depositing method and its annealing condition.
TABLE 1 ______________________________________ (Sensing characteristics of ion sensing membranes of the pH-ISFETs) sensing membrane characteristics SiO.sub.2 Si.sub.3 N.sub.4 Al.sub.2 O.sub.3 ZrO.sub.2 Ta.sub.2 O.sub.5 ______________________________________ possible 4-10 1-13 1-13 0-8 1-13 range of pH measurement sensitivity 25-35 46-56 53-58 50-58 56-58 (mV/pH) response time 1 &lt;0.1 &lt;0.1 &gt;0.1 &lt;0.1 (95%, sec) long-term unstable 3.0 0.8 stable 0.2 stability (mV/h, pH 7) ______________________________________
Conventional methods of depositing the Ta.sub.2 O.sub.5 thin film is divided roughly into two groups; RF sputtering process and chemical vapor deposition (CVD) process.
Ta.sub.2 O.sub.5 film can be formed by thermal CVD, plasma enhanced CVD (PECVD), photo CVD or low pressure CVD (LPCVD) process among the various CVD processes.
Generally, the CVD processes provide such advantages as the formation of the film in that the film is excellent in its stoichiometric composition and in its productivity. However, the equipment for this process is very expensive and complicated, and thus it is difficult to determine the optimum deposition parameters. Further, it has the disadvantage that the gas to be used is costly and consuming.
On the other hand, the equipment for the RF sputtering process is relatively inexpensive and easy operate according to its simple structure, but this process has the disadvantages that it is difficult to effect the desired stoichiometric composition and to avoid the damage on the film surface caused by its plasma process and resulting lower productivity.
Japanese Patent Laid-open No. 62-245956 published on Oct. 27, 1987 discloses a pH-ISFET sensor in which Ta.sub.2 O.sub.5 hydrogen ion sensing membrane is deposited by the RF sputtering process.
According to the above-mentioned invention, the Ta.sub.2 O.sub.5 thin film of the ISFET is deposited by the RF sputtering process, but it is difficult to improve the stoichiometric composition ratio of tantalum and oxygen and this causes a decrease in the sensitivity of the pH-ISFET sensor. In that invention, after deposition step, the film is annealed at a temperature of 275.degree.-300.degree. C. in an inert gas (e.g. nitrogen or argon gas) ambience for 10 minutes. However, its annealing temperature is too low to obtain the desired annealing effect. Also, the Ta.sub.2 O.sub.5 film lacks oxygen, which means that it suffers from a non-stoichiometric composition ratio. Accordingly, it gives a definitive influence to the sensitivity and stability of the sensor as the sensing mechanism of the ISFET is a kind of site binding depending upon the condition of the sensing membrane surface.
Therefore, under the above-mentioned annealing condition, the sensors undergo undesirable influences in the sensitivity and the stability thereof. Further, during the deposition of the Ta.sub.2 O.sub.5 thin film, the bonding region of the ISFET except its gate region is covered with a mask and then sputtered. Such process utilizing the mask is not a whole wafer process by photo etching, but is carried out manually, and then it is troublesome in productivity and standardization.